What's less known, the city-owned utility has turned to other energy users to meet its own goals. It is paying the tab for them to reduce emissions.

City Light is buying low-emission biodiesel fuel for use in state ferries and in Metro buses, and it's hooking up cruise ships to electricity in port so they can shut off diesel engines.

City Light gets "credit" for the reduction of greenhouse gases released into the atmosphere. In November, the utility even signed a contract with DuPont Fluorochemicals for offset purchases of energy savings.

Enter the plaintiffs, a collection of two former assistant city attorneys and two ex-City Light employees who have scored on past challenges to how the utility spends ratepayers' money.

They successfully took on the city for transferring the cost of operating streetlights from Seattle's general fund to City Light. Charging that this amounted to an illegal tax, they forced the city to reimburse City Light to the tune of $25 million -- plus paying plaintiffs' legal costs.

The plaintiffs mounted a second fight, challenging City Light's "One Percent for Art" program as inconsistent with the utility's mission.

They've now amended their complaint again to challenge City Light's offset program, from biodiesel purchases to the deal with DuPont.

The plaintiffs have even raised a comparison to the Civil War policy in which male citizens were able to pay to have others take their place in the Union army.

It's an intriguing case, and a big deal.

"The state is moving down the biodiesel road and here appears a major bump: Metro and Washington State Ferries would not have switched over to biodiesel if City Light had not borne the costs," said professor Michael Robinson-Dorn of the University of Washington Law School's environmental law clinic.

Joel Connelly has been a staff columnist for more than 30 years. He comments regularly on politics and public policy.

Or, as noted in a recent Environmental Law Review article, "To our knowledge, utility purchases of greenhouse gas offsets have never before been challenged in court."

The plaintiffs' arguments have a straightforward appeal.

"The only thing City Light has received or will receive in return for these expenditures is some fictional 'credit' toward meeting the city's voluntary, self-imposed commitment to reduce greenhouse gas emissions as a means of combating global warming," they argue.

"None of these programs generates, conserves or distributes a single watt of electricity to any City Light customer, or reduced a single molecule of any greenhouse gas generated by any City Light activity."

In ruling for City Light last September, however, King County Superior Court Judge Sharon Armstrong saw it another way, saying City Light has control over how to reduce its emissions.

"It can do so by managing its own facilities, its own producing facilities, or it can spend money to have its emissions, its contribution reduced by someone else," Armstrong wrote.

"This all makes sense only because of the unusual nature of the greenhouse gas canopy; the fact is that it is an envelope around the entire globe, that it's not localized; it does circulate."

An ounce, a pound or a ton means that quantity of pollutants is not warming the Earth. Measures taken by City Light, Metro or DuPont all have the same impact.

Seattle City Light's power comes from Northwest rivers.

As UW climatologists have outlined, global warming means big changes in these parts. The winter snowpack will be much smaller and melt more quickly. Glaciers will disappear, or retreat drastically.

The snowpack fills reservoirs behind dams on the Skagit, Pend Oreille and Columbia rivers. Rivers are fed in the summer by melt off ice fields in the Canadian Rockies, Cascades, Selkirk and Purcell mountains.

The buildup of greenhouse gases threatens not only the region's environment, but a key foundation of its economy.

In the streetlight case, our Supreme Court sided with the plaintiffs against City Light.

Here's a suggestion for the Supremes: Instead of the Temple of Justice in Olympia, they ought to hear this case up at Colonial Creek Campground just off the North Cascades Highway.

The justices could hitch up their robes and hike down to where -- all summer -- the green waters of Thunder Creek rush into Diablo Lake.

Melt from a bevy of great glaciers -- the Logan, Boston, Inspiration, Klawatti and Neve -- supplies about 45 percent of the flow (down from 70 percent a century ago), water that generates kilowatts at two of City Light's Skagit River dams.

Seattle City Light's efforts to cut greenhouse gas emissions are not an indulgence. They're a necessity.